Radio altimeters determine altitude by transmitting a radio signal to the ground and timing the delay of the return echo. Most general aviation radio altimeters transmit a frequency modulated continuous wave (FMCW) at 4.2 GHz to 4.4 GHz. The continuous wave is usually frequency modulated such that the frequency increases linearly, then decreases linearly. The linearity and the slope of the modulation are key parameters that affect the accuracy of the radio altimeter. The transmitted frequency modulated radio signal must be extremely linear in frequency as it varies in time. The frequency deviation rate must occur at a precise slope (Hz per foot of altitude rate).
Modulation linearity is very difficult to maintain. The linearity is adjusted during production by means of variable circuit elements. Linearity may also be maintained through using close tolerance circuit components to limit variability. Current radio altimeter calibration technology requires delay lines to help maintain the desired slope during operation. To be effective, the delay line must be manufactured to close tolerances using close tolerance circuit elements. Delay line installation is labor intensive. Current radio altimeter calibration technology does not provide any means for dynamically correcting modulation non-linearity's.